00 V70 XC SE What's up with the security screws on the MAF?

[goVolvo]

Bought a new MAF and what a difference! Easy starting, smooth running.

But what's up with the 5-point security screws that mount the sensor? Why does Volvo think its so important to not unscrew it? It's just a sensor inserted into a plastic collar.

I ask because my replacement MAF assembly has a diameter that is a bit too large to fit into my filter housing and air duct (I had to duct tape it in place). I want to put the new sensor into the old assembly, but there are those darn 5-point security screws. Why does Volvo think that never removing that part is so critical?

Last week I found a set of 7 or 8 hollow/secure torx bits on clearance sale at OReilly. 3 bucks, what a relief!
The one I see has 6 lobes inside, not 5.

[northernlights]

I think they are factory calibrated to match the housing, because the output signal is critical at low flow. Given production tolerances and the number of variables being measured I would be amazed if they can be assembled without any calibration.

The old OEM ones (Bosch HFM2 and HFM5) seem to all have a small dab of epoxy or something similar on the side. My suspicion is that is related to factory calibration, but that is pure speculation on my part.

[instarx]

I think they are factory calibrated to match the housing, because the output signal is critical at low flow. Given production tolerances and the number of variables being measured I would be amazed if they can be assembled without any calibration.

But there is absolutely no way to adjustment them. The sensor base sits on a flat flange on the collar so depth isn't adjustable. Neither is tilt, for the same reason. Rotation isn't adjustable because of the two screws. If it can't be adjusted it can't be calibrated. If I could see even one parameter that could be changed when you insert that sensor into the collar I might go for "calibrated at the factory to fit the collar", but there isn't.

My idea of how it goes: the sensor manufacturer makes a bunch of sensors and tests each one to make sure it is within specs electrically. Then they open a box with a bunch of identical collars and put them together. The design of the collar and sensor is such that they are automatically aligned properly.

I think its just a Federal regulation that emission items need to be tamper resistant, hence the security screws.

[erikv11]

You're lucky that with aftermarket maf your car even runs. ...

You can cay that again.

[northernlights]

I think they are factory calibrated to match the housing, because the output signal is critical at low flow. Given production tolerances and the number of variables being measured I would be amazed if they can be assembled without any calibration.

But there is absolutely no way to adjustment them. The sensor base sits on a flat flange on the collar so depth isn't adjustable. Neither is tilt, for the same reason. Rotation isn't adjustable because of the two screws. If it can't be adjusted it can't be calibrated. If I could see even one parameter that could be changed when you insert that sensor into the collar I might go for "calibrated at the factory to fit the collar", but there isn't.

You have to think in terms of calibration, which is not the same as physically adjusting or aligning things. Calibration is common practice with instrumentation, and may partly explain the seemingly absurd prices for the OEM parts.

Here are a two pages from a SAE textbook (Mixture Formation in Spark-Ignition Engines) that show details of the Bosch HFM2 sensor, which is what was used in the Motronic 4.3 and 4.4 for Volvo.

Unfortunately, this book was originally in German, so things are not always 100% clear in the English version. But, you will note that in the last paragraph of the first page they refer to a trim resistor, which I believe corresponds to R1 on the second page.

Note also on the second page there are three symbols that possibly look like trim controls. If they are trim controls (think of a real or virtual potentiometer) how exactly this would be accomplished I do not know.

As I said before, this is conjecture on my part because I have never worked for a company which actually manufactured these things. But, having worked with instrumentation, if I was producing an instrument that needed to reliably report accurate data I would provide some means of factory calibration. This might be as simple as a single point calibration, i.e. a certain voltage for a known airflow under controlled conditions.

On cold start the O2 sensor doesn't work on these cars, so the airflow meter is the best source of data upon which to base the fueling rate. Cold start emissions are the hardest part to satisfy, enhansing the need for an accurate air flow rate.

[instarx]

Don't get me wrong. I am not saying that these sensors aren't calibrated - far from it. I'm saying that they aren't calibrated to the individual ducts they are put into. That was the original objection - that I couldn't put a new sensor into my measurement channel (duct) because it wouldn't be calibrated to it.

Once a sensor is verified to be in spec at the manufacturer there is no adjustment required. How could there be - I do not see any way to calibrate it. Look at the circuit diagram on the second page above. It is really a simple device - there is no memory, no potentiometer, no variable resister, not even a mechanical airflow deflector. That triangular thing is an amplifier (if it were adjustable there would be more than three leads) and the rest of the components are non-variable resistors. There is simply no way to calibrate this thing. It will either be in spec or it won't.

[northernlights]

Agreed as far as the op-amp goes, but what do you think the 'T'shape below R(s) on the left figure is, and the two similar shapes on the right figure?

For that matter, if Bosch was making the entire sensor, and possibly calibrating it, why wouldn't they calibrate the assembly vs. the insert?

[Atis]

Some additional material:

[instarx]

Agreed as far as the op-amp goes, but what do you think the 'T'shape below R(s) on the left figure is, and the two similar shapes on the right figure?

For that matter, if Bosch was making the entire sensor, and possibly calibrating it, why wouldn't they calibrate the assembly vs. the insert?

I believe those are grounds, but I wouldn't swear to it.

I really doubt they calibrate them as such. They wouldn't put each one in a wind tunnel and test it, for example. They probably plug them into a test circuit and simply see what voltages they produce based on various inputs. If they are within specs they pass.

[Paul-93-850]

Use a little Dremel cutting wheel and cut a straight slot across the head of the screw. Remove the screws with a straight standard slot screwdriver.